In a tandem pump, an intermediate side plate is disposed between two rotors. Reducing the gaps between each rotor and the side plate to reduce the flow rate of oil leaking through these gaps is important to enhance the volumetric efficiency. Therefore, in a known structure, the intermediate side plate is divided into two side plate portions so that a pressure chamber is formed between them. Oil delivered by rotation of one rotor is introduced into this pressure chamber to resiliently deform the side plate portions toward their respective opposite rotors, thus narrowing the aforementioned gaps.
In this conventional tandem pump, the grooves formed in the rotors for receiving vanes are made uniformly in width so that the pumps produce pressure equally at the beginning of rotation. However, the widths of the grooves are not uniform due to machining error. Consequently, when the pump is started, the vanes of the rotor in the first pump chamber may come out earlier than the vanes of the rotor in the second pump chamber. This phenomenon becomes conspicuous at low temperatures where the viscosity of the working oil is large. If such a phenomenon takes place, the pressure of the oil admitted into the pressure chamber causes the side plate portion opposite to the rotor inside the second pump chamber to deflect excessively toward the rotor because the pressure inside the second pump chamber is low. The gap between the rotor and the side plate portion is then lost. As a result, a sufficient flow of lubricating oil fails to be supplied. Hence a seizure occurs.